1. Field of the Invention
The present invention relates to hydrophobic inorganic fine particles for use in an image-forming method such as an electrophotographic method, an electrostatic printing method or a toner jet method, and a toner containing the hydrophobic inorganic fine particles.
2. Description of the Related Art
It has been generally known that inorganic fine particles are externally added to toner particles for the following purpose of adjusting the charging performance, flowability and the like of the toner so that a good developing property can be obtained. Wet process silica, dry process silica, titanium oxide, alumina or the like has been conventionally used as hydrophobic inorganic fine particles, and a proposal has been made that inorganic fine particles is subjected to hydrophobic treatment in order that the environmental stability of the charging of toner may be improved.
Surface treatment with, for example, a silane compound, a silazane compound or a silicone oil has been performed as hydrophobic treatment for those inorganic fine particles.
For example, an approach has been adopted in which a reaction treatment agent such as dimethyldichlorosilane or hexamethyldisilazane, and hydrophilic inorganic fine particles are brought into contact with each other, and a hydroxyl group present on the surface of the hydrophilic inorganic fine particle and the treatment agent are allowed to react with each other so that the hydrophilic inorganic fine particle can be made hydrophobic.
However, a hydrophobic group introduced to the surface of the inorganic fine particle by such a treatment agent has a large molecular size, and the hydrophobic group is unable to reach an adjacent hydroxyl group owing to steric hindrance. As a result, a hydroxyl group is left which does not react and hence is not made hydrophobic, so sufficient hydrophobicity cannot be imparted to the inorganic fine particle in some cases.
In addition, it is difficult to uniformly subject the surfaces of inorganic fine particles to surface treatment with silicone oil because the silicone oil has a relatively high molecular weight and high viscosity.
In particular, when toner is stored in a high-temperature, high-humidity environment over a long time period before being used, the external addition of inorganic fine particles which have not been subjected to sufficient hydrophobic treatment to toner may involve such a problem that the flowability and charging performance of the toner is lowered owing to inorganic fine particles having absorbed moisture, and hence the quality of an image formed from the toner deteriorates. Further, when such a stored toner is used for a long time, the toner deteriorates so that its flowability and charging performance is remarkably lowered. As a result, the toner (transfer residual toner) remaining on a photosensitive member without being transferred onto a transfer material such as paper is not collected in a cleaning container, and drips to cause image defects or contamination in an image-forming apparatus in some cases.
Japanese Patent Publication No. H07-113783 discloses a negatively chargeable electrophotographic developer characterized by containing: a negatively chargeable silicate fine powder having been treated with hexamethyldisilazane or a silane coupling agent, and then further treated with a silicone oil; and toner.
According to the method, the degree of hydrophobicity of the silica increases because a hydroxyl group which was not treated with hexamethyldisilazane or the silane coupling agent is made hydrophobic by the silicone oil. However, most of the silicone oil in the treatment is not immobilized onto the surface of the silica, and so, a problem is raised in some case in that when the silica is stored under a high-temperature, high-humidity environment for a long time period in a state of being externally added to the toner, the toner is apt to absorb moisture so that its charging is inhibited.
Japanese Patent Application Laid-Open No. 2002-256170 discloses a hydrophobic silica powder obtained by subjecting a raw silica powder to surface-treatment with a polysiloxane and a trimethylsilylating agent, the hydrophobic silica powder being characterized in that: 0.3 to 1.5 trimethylsilyl groups are present per 1 nm2 of the surface area of the raw silica powder; and the polysiloxane adheres in an amount of A/20 to A/5 parts by mass (where A represents the specific surface area (m2/g) of the raw silica powder) with respect to 100 parts by mass of the raw silica powder. In addition, a method of producing the hydrophobic silica powder is disclosed which includes treating a silica powder with a polysiloxane and then with a trimethylsilylating agent.
Japanese Patent Application Laid-Open No. 2004-168559 discloses a highly dispersible, highly hydrophobic silica powder characterized in that: the silica powder has a hydrophobicity rate of 95% or more and a hydrophobicity degree of 76% or more; and the silica powder has such a distribution density that its particle sizes are concentrated in a range of 10 to 70 μm. In addition, a method of producing the highly dispersible, highly hydrophobic silica powder is disclosed which includes the steps of: subjecting silica to primary surface treatment with a silicone oil treatment agent; after the primary surface treatment, disintegrating the treated product; and after the disintegration, subjecting the disintegrated products to secondary surface treatment with an alkylsilazane treatment agent.
In these methods, silica is subjected to surface treatment with a silicone oil, and is then subjected to hydrophobic treatment with a treatment agent such as hexamethyldisilazane, and so, the silicone oil can be easily immobilized onto the silica surfaces, and the hydrophobicity degree of silica can be increased in a state that the amount of liberated silicone oil is reduced. However, the amount of liberated silicone oil which is not immobilized onto the surfaces of the silica particles becomes extremely small, and so, problems are liable to occur such that contamination of a contact charging member due to adhesion of the silica powder to the member, or the melt adhesion of a toner containing the silica powder to a photosensitive member.
Japanese Patent Application Laid-Open No. 2007-176747 discloses a surface-coated silica obtained by subjecting silica to coating treatment with two or more different types of silicone oils, the surface-coated silica being characterized in that: the amount of silicone oils extracted with chloroform accounts for 5 to 95 mass % of the amounts of all the coated silicone oils; and at least one type of silicone oil is not included in the extracted silicone oils.
In this method, the abundance of liberated silicone oil on the silica surfaces can be controlled, but it is difficult for silicone oil to penetrate fine irregularities on the silica surfaces because its molecular weight is larger than that of a reaction treatment agent such as dimethyldichlorosilane or hexamethyldisilazane. Accordingly, hydroxyl groups which have not been subjected to hydrophobic treatment are apt to remain on the silica surfaces, so that when a toner is stored in a high-temperature, high-humidity environment over a long time period before being used, inorganic fine particles absorb moisture to inhibit charging and to cause image quality deteriorate in some cases.
Thus, it is desired that inorganic fine particles are developed which can provide toner with charging performance and flowability which are stable irrespective of a circumstance under which the toner is stored and an environment in which the toner is used.